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An investigation of the SO2Ag surface reaction using secondary ion mass spectrometry
Volume
27, number
3
CHEMICAL
AN INVESTIGATION
1 August
LETTERS
OF THE S02/Ag
USING SECONDARY
Department
PHYSICS
SURFACE
1974
REACTION
ION MASS SPECTROMETRY
M. BARBER, P. SHARPE and J.C. VICKERMAN of Chemistry, The University of Manchester Institute of Science and Technology, Manchester M60 lQD, UK Received
13 May 1974
The interaction of SO* and of SOz + O2 mixtures with Ag surfaces has been spectroscopy. The SOz/Ag system gave spectra which were largely composed of dicative of the formation of S and SO4 species in the reaction. In the case of the species were similar to those in the previous reaction, the differences being that, AgSO: disappeared to be replaced by AgzO+ and AgO+. These observations are evidence from other techniques and workers in the field.
1. Introduction Little work has been reported in the literature on the Ag/SO, surface reaction [ 11, even though silver is an important catalyst in the chemical industry and is selectively poisoned with SO,. It is used extensively in electrical components, which may have to operate in atmospheric conditions where SO, is a trace pollutant thus adversely affecting their performance. However, work has been reported [2,3] on the reaction of SO2 with other metals, e.g., Fe and Ni. It would seem important, therefore, to investigate the initial stages of the Ag/SO, reaction, under clean vacuum conditions and with a technique which can give information about the species formed on the surface. To this end we have undertaken a preliminary study of the interaction of SO, with a pure polycrystalline Ag foil using secondary ion mass spectrometry to examine the types of intermediates formed.
investigated using secondary ion mass AgSO:, AgSOz and AgzS+ ions, in(SO2 + Oz)/Ag system the initial as the reaction proceeded, AgSl and discussed in the light of supporting
Base pressures in the analysis chamber of 1O-lo torr were routinely attainable. Sample cleaning, by argon ion etching, was carried out in a separately pumped vessel in which similar base pressures could be activated. In this series of experiments high purity polycrystalline Ag foil was used and 99.9 % pure dried SO2 employed in the reaction. Two experiments were carried out. The first was to study the interaction of SO, with the Ag foil in situ at equilibrium pressures of = lop5 torr, analysis being carried out as the SO2 was streamed over the Ag foil. The second study was to introduce 0, into the SO, stream in large excess, the final composition of the mixture being 10 % SO, - 90 % 0,. All the reactions were carried out with the Ag foil heated to x 350°C.
3. Results 3. I. The SO,/Ag
reaction
2. Experimental The secondary ion mass spectrometer (SIMS) was constructed by Vacuum Generators and was similar in principle to that described by Benninghoven [4]. The instrument has been described in more detail elsewhere [51. 436
The pure dried SO2 was passed over the foil at 350” at a pressure of lop5 torr. The initial secondary ion mass spectrum contained AgSOi and a relatively large increase was noted in the heights of both the Ag+ and Agz peaks. As the reaction proceeded, the emergence of peaks due to AgSO$ and Ag2S+ was observed.
Volume
27, number
3
CHEMICAL
PHYSICS
LETTERS
I
August
1974
100
Int
50
1.0
0.5
Fig.
15
1
These continued to increase at a rapid rate (throughout the course of the reaction). The AgSO; peak, whilst substantially increasing in intensity, did so at a much slower rate. Towards the end of the reaction runs, other peaks began to appear in the system, namely AgSO+ , AgSO; , AgS+ and Ago”. These then increased slowly, but remained minor components of the spectrum. The behaviour of these peaks is shown qualitatively in fig. 1. As will be noted the major effect is the production of AgSO$ and Ag2S’; however, owing to the limited mass range of the quadrupole mass spectrum a search for species such as Ag2S0$ was not possible. 3.2. The (SO,/O,)/Ag
reaction
The gas mixture contained about 10 % SO, and 90 % 0,. This was passed over the heated Ag foil (350°C) at a pressure of lop5 torr and the secondary ion mass spectra taken as the reaction proceeded. The initial spectra contained similar ions to those obtained from the pure SO,/Ag system, namely, AgSOz, AgSOi and AgzS+. This was followed by the usual increase in Ag2S+ and AgSO;f and the emergence of the AgS+ species found for the previous system. However, as the reaction continued, a rapid decrease in AgSO$ occurred. This was followed by a decrease in the AgzS+ peak height and the emergence and increase of Ag2 O+ .During this period the AgSOs peak heights remained substantially constant. The peak
Fig. 2
height as time changes are shown qualitatively
in fig. 2.
4. Discussion Both Lassiter [l] and Saleh [2] from their observations agree that SO, is dissociatively adsorbed over Ag and other metals at elevated temperatures. The Auger spectra of such systems [l] show clearly the build-up of S on the surface. Interestingly, however, this technique does not reveal the presence of oxygen on the surface. One interpretation which has been put on this, is that the SO, complexes are oxygen bonded to the surface, and hence the possibility exists for shadowing of the oxygen by the bulkier sulphur. Another explanation is that the oxygenated species are readily desorbed by electron bombardment, which may not be the case for the more thermodynamically stable sulphide system. From the SIMS results, however, whilst agreement is obtained over the formation of S on the surface, there is no doubt that SO, and SO, species exist on the surface. It is interesting to note that the S formed is largely associated with two silver atoms, with very little contribution from a single silver atom. Supporting evidence for the formation of oxygenated species on the surface comes from the work of Blyholder and Cagle [3] who have observed the IR spectrum of SO, adsorbed at 25°C on Fe and Ni, and interpret the results as indicative of the formation of complexed SO, units. Unfortunately the observation 437
Volume
27, number
3
CHEMICAL
PHYSICS
of M-S bond formation was outside the wavelength region of their instrument. From our preliminary results on the SO,/Ag system, together with the above supporting evidence, we can tentatively describe the initial processes as adsorption of the SO2 on single Ag atom sites SO2 (gas) --+ SO, (ads) , followed by dissociative oxidation and SO,
of this to form S
SO, (ads) + SO2 (ads) --+ S(ads) + SO, (ads) The free S can then bond predominantly to two Ag atoms and the SO4 complex to one. There is evidence from the sign of the charge of the sputtered ions that these species are not the usual SOi- and S2-; according to Benninghoven [6] the sign of the sputtered cluster being the same as the sign of the sum of the charges of the units existing in the surface. Thus for monovalent Ag, one might not have expected AgSO, as a positive species. This is once again borne out by the IR evidence, the lowering of the frequencies suggesting that it is not SOi- as such, but more likely a complexed SO, molecule which is present. The results from the 02/S02 mixtures indicate that there is rapid removal of S and SO, from the surface. Buckley [7] has reported that S films on the surface of such metals as Cu and Al are removed at room temperature by oxygen, which supports our own observations on silver. It has been suggested [3] that the
438
LETTERS
1 August
1974
SO, complex could well be an intermediate in SO3 formation, for example the oxidation of CO to CO2 over metal catalysts is thought to proceed through a CO, surface complex. Thus the surface is thought to promote the formation of a complex similar to the highest anion oxide which can further decompose to give a gas phase oxide of lower oxygen to other atom ratios. This would account in our case for the emergence of small intensity AgSO; peaks. Further work is now in progress to investigate this reaction using SIMS.
Acknowledgement Our thanks are due to Vacuum Generators Ltd. for the generous loan of equipment on which this investigation was carried out, and also to Dr. Stacey of I.C.I. for helpful discussions.
References [l] W.S. Lassiter, J. Phys. Chem. 76 (1972) 1289. [2] J.M. Saleh, Trans. Faraday Sot. 64 (1968) 796. [3] G. Blyholder and G.W. Cagle, Environ. Sci. Technol. 5 (1971) no. 2. [4] A. Benninghoven, Surface Sci. 35 (1973) 427. [S] M. Barber and J.C. Vickerman, to be published. [6] A. Benninghoven and A. Mtiller, Surface Sci. 39 (1973) 416. [7] D.H. Buckley, NASA Tech. Rept. D.7340.
27, number
3
CHEMICAL
AN INVESTIGATION
1 August
LETTERS
OF THE S02/Ag
USING SECONDARY
Department
PHYSICS
SURFACE
1974
REACTION
ION MASS SPECTROMETRY
M. BARBER, P. SHARPE and J.C. VICKERMAN of Chemistry, The University of Manchester Institute of Science and Technology, Manchester M60 lQD, UK Received
13 May 1974
The interaction of SO* and of SOz + O2 mixtures with Ag surfaces has been spectroscopy. The SOz/Ag system gave spectra which were largely composed of dicative of the formation of S and SO4 species in the reaction. In the case of the species were similar to those in the previous reaction, the differences being that, AgSO: disappeared to be replaced by AgzO+ and AgO+. These observations are evidence from other techniques and workers in the field.
1. Introduction Little work has been reported in the literature on the Ag/SO, surface reaction [ 11, even though silver is an important catalyst in the chemical industry and is selectively poisoned with SO,. It is used extensively in electrical components, which may have to operate in atmospheric conditions where SO, is a trace pollutant thus adversely affecting their performance. However, work has been reported [2,3] on the reaction of SO2 with other metals, e.g., Fe and Ni. It would seem important, therefore, to investigate the initial stages of the Ag/SO, reaction, under clean vacuum conditions and with a technique which can give information about the species formed on the surface. To this end we have undertaken a preliminary study of the interaction of SO, with a pure polycrystalline Ag foil using secondary ion mass spectrometry to examine the types of intermediates formed.
investigated using secondary ion mass AgSO:, AgSOz and AgzS+ ions, in(SO2 + Oz)/Ag system the initial as the reaction proceeded, AgSl and discussed in the light of supporting
Base pressures in the analysis chamber of 1O-lo torr were routinely attainable. Sample cleaning, by argon ion etching, was carried out in a separately pumped vessel in which similar base pressures could be activated. In this series of experiments high purity polycrystalline Ag foil was used and 99.9 % pure dried SO2 employed in the reaction. Two experiments were carried out. The first was to study the interaction of SO, with the Ag foil in situ at equilibrium pressures of = lop5 torr, analysis being carried out as the SO2 was streamed over the Ag foil. The second study was to introduce 0, into the SO, stream in large excess, the final composition of the mixture being 10 % SO, - 90 % 0,. All the reactions were carried out with the Ag foil heated to x 350°C.
3. Results 3. I. The SO,/Ag
reaction
2. Experimental The secondary ion mass spectrometer (SIMS) was constructed by Vacuum Generators and was similar in principle to that described by Benninghoven [4]. The instrument has been described in more detail elsewhere [51. 436
The pure dried SO2 was passed over the foil at 350” at a pressure of lop5 torr. The initial secondary ion mass spectrum contained AgSOi and a relatively large increase was noted in the heights of both the Ag+ and Agz peaks. As the reaction proceeded, the emergence of peaks due to AgSO$ and Ag2S+ was observed.
Volume
27, number
3
CHEMICAL
PHYSICS
LETTERS
I
August
1974
100
Int
50
1.0
0.5
Fig.
15
1
These continued to increase at a rapid rate (throughout the course of the reaction). The AgSO; peak, whilst substantially increasing in intensity, did so at a much slower rate. Towards the end of the reaction runs, other peaks began to appear in the system, namely AgSO+ , AgSO; , AgS+ and Ago”. These then increased slowly, but remained minor components of the spectrum. The behaviour of these peaks is shown qualitatively in fig. 1. As will be noted the major effect is the production of AgSO$ and Ag2S’; however, owing to the limited mass range of the quadrupole mass spectrum a search for species such as Ag2S0$ was not possible. 3.2. The (SO,/O,)/Ag
reaction
The gas mixture contained about 10 % SO, and 90 % 0,. This was passed over the heated Ag foil (350°C) at a pressure of lop5 torr and the secondary ion mass spectra taken as the reaction proceeded. The initial spectra contained similar ions to those obtained from the pure SO,/Ag system, namely, AgSOz, AgSOi and AgzS+. This was followed by the usual increase in Ag2S+ and AgSO;f and the emergence of the AgS+ species found for the previous system. However, as the reaction continued, a rapid decrease in AgSO$ occurred. This was followed by a decrease in the AgzS+ peak height and the emergence and increase of Ag2 O+ .During this period the AgSOs peak heights remained substantially constant. The peak
Fig. 2
height as time changes are shown qualitatively
in fig. 2.
4. Discussion Both Lassiter [l] and Saleh [2] from their observations agree that SO, is dissociatively adsorbed over Ag and other metals at elevated temperatures. The Auger spectra of such systems [l] show clearly the build-up of S on the surface. Interestingly, however, this technique does not reveal the presence of oxygen on the surface. One interpretation which has been put on this, is that the SO, complexes are oxygen bonded to the surface, and hence the possibility exists for shadowing of the oxygen by the bulkier sulphur. Another explanation is that the oxygenated species are readily desorbed by electron bombardment, which may not be the case for the more thermodynamically stable sulphide system. From the SIMS results, however, whilst agreement is obtained over the formation of S on the surface, there is no doubt that SO, and SO, species exist on the surface. It is interesting to note that the S formed is largely associated with two silver atoms, with very little contribution from a single silver atom. Supporting evidence for the formation of oxygenated species on the surface comes from the work of Blyholder and Cagle [3] who have observed the IR spectrum of SO, adsorbed at 25°C on Fe and Ni, and interpret the results as indicative of the formation of complexed SO, units. Unfortunately the observation 437
Volume
27, number
3
CHEMICAL
PHYSICS
of M-S bond formation was outside the wavelength region of their instrument. From our preliminary results on the SO,/Ag system, together with the above supporting evidence, we can tentatively describe the initial processes as adsorption of the SO2 on single Ag atom sites SO2 (gas) --+ SO, (ads) , followed by dissociative oxidation and SO,
of this to form S
SO, (ads) + SO2 (ads) --+ S(ads) + SO, (ads) The free S can then bond predominantly to two Ag atoms and the SO4 complex to one. There is evidence from the sign of the charge of the sputtered ions that these species are not the usual SOi- and S2-; according to Benninghoven [6] the sign of the sputtered cluster being the same as the sign of the sum of the charges of the units existing in the surface. Thus for monovalent Ag, one might not have expected AgSO, as a positive species. This is once again borne out by the IR evidence, the lowering of the frequencies suggesting that it is not SOi- as such, but more likely a complexed SO, molecule which is present. The results from the 02/S02 mixtures indicate that there is rapid removal of S and SO, from the surface. Buckley [7] has reported that S films on the surface of such metals as Cu and Al are removed at room temperature by oxygen, which supports our own observations on silver. It has been suggested [3] that the
438
LETTERS
1 August
1974
SO, complex could well be an intermediate in SO3 formation, for example the oxidation of CO to CO2 over metal catalysts is thought to proceed through a CO, surface complex. Thus the surface is thought to promote the formation of a complex similar to the highest anion oxide which can further decompose to give a gas phase oxide of lower oxygen to other atom ratios. This would account in our case for the emergence of small intensity AgSO; peaks. Further work is now in progress to investigate this reaction using SIMS.
Acknowledgement Our thanks are due to Vacuum Generators Ltd. for the generous loan of equipment on which this investigation was carried out, and also to Dr. Stacey of I.C.I. for helpful discussions.
References [l] W.S. Lassiter, J. Phys. Chem. 76 (1972) 1289. [2] J.M. Saleh, Trans. Faraday Sot. 64 (1968) 796. [3] G. Blyholder and G.W. Cagle, Environ. Sci. Technol. 5 (1971) no. 2. [4] A. Benninghoven, Surface Sci. 35 (1973) 427. [S] M. Barber and J.C. Vickerman, to be published. [6] A. Benninghoven and A. Mtiller, Surface Sci. 39 (1973) 416. [7] D.H. Buckley, NASA Tech. Rept. D.7340.